The Latest on H1N1 Swine Flu

A CDC analysis shows that pregnant women are more likely than the general population to develop severe disease after infection with the pandemic H1N1 swine flu virus. They are four times more likely to be hospitalized, with an unusually high death rate.

Although media reports have focused on deaths among pregnant women with underlying disease, most pregnant women who have died of swine flu were healthy when they caught the pandemic virus.

Even so, the findings do not mean that all pregnant women will suffer severe disease, says study leader Denise J. Jamieson, MD, MPH, an obstetrician-gynecologist at the CDC's Women's Health and Fertility branch.

"Most women who become ill with flu are going to have mild disease just like in the general population," Jamieson tells WebMD. "But it does seem pregnant women are at increased risk of severe illness and death. So the proportion of deaths among pregnant women is larger than you would expect."

The CDC has details on 266 of the 305 U.S. swine flu deaths reported as of July 29. Fifteen of these 266 deaths -- about 6% -- were among pregnant women. Most of the deaths were among women in their third trimester.

Pandemic Flu Deaths in Pregnant Women Not New

It's not a new phenomenon:

During seasonal flu epidemics, pregnant women -- especially those in the third trimester -- are far more likely to be hospitalized for lung and heart disease than women who recently delivered a child (considered the closest comparison group).

In a study of 1,350 women who had flu during the 1918 pandemic, about half the women got pneumonia and about half of the women with pneumonia died -- a case-fatality rate of 27%. Women in their third trimester were particularly vulnerable.

During the 1957 flu pandemic, pregnant women accounted for half of the flu deaths among Minnesota women of reproductive age.

There's also a risk to the fetus. In past pandemics, there were high rates of stillbirth, spontaneous abortion, and premature delivery among pregnant women who had the flu. Flu comes with fever, which can result in brain damage to the fetus.

In the current swine flu pandemic, Jamieson says, it's too soon to know much about fetal outcome. But she says women with swine flu do seem to be at high risk of premature delivery.

But the major risk is to the pregnant woman herself. That risk is based on her pregnancy and not on other risk factors. "Pregnant women are at risk just by virtue of being pregnant," Jamieson says.

Why? It's not entirely clear, but there are two likely mechanisms:

As pregnancy evolves, a woman's diaphragm is pushed upward and she has decreased lung capacity. This makes respiratory disease more dangerous.

During pregnancy, a woman's immune system shifts away from the kinds of immune responses most effective in battling viral infections. This makes her more susceptible to some viral diseases such as flu.

CDC to Pregnant Women with Flu: Take Tamiflu or Relenza

All of the pregnant women who died of pandemic H1N1 swine flu had another thing in common: They, or their doctors, waited too long to start them on anti-flu drugs.

Tamiflu and Relenza both fight swine flu. These drugs work far better when started within two days of the first flu symptoms.

Many women -- and many doctors -- are hesitant to start medications during pregnancy. But when flu symptoms strike, the antiviral drugs Tamiflu and Relenza pose vastly less risk than the flu itself.

"Clinicians who take care of pregnant women seem hesitant to start antivirals," Jamieson says. "It has been an issue of prompt antivirals vs. concerns about the fetus. But we are recommending that women be treated, because the benefits outweigh the risks of the drug."

Mark Phillippe, MD, chairman of obstetrics, gynecology, and reproductive sciences at the University of Vermont, recently wrote an editorial warning doctors to offer pregnant women aggressive treatment if they have flu symptoms. He was not involved in the CDC report.

"I certainly agree with the CDC recommendation," Phillippe tells WebMD. "The risk of becoming seriously ill and of dying is significantly higher for pregnant women than for the rest of the population. The big advantage we have over previous pandemics is we have an opportunity to save lives by early treatment. Based on the case reports so far, most of the women who died had a delay in treatment."

Jamieson says that a pregnant woman who develops flu-like symptoms should call her doctor right away. She should not go directly to her doctor's office to avoid exposing other pregnant women to swine flu. If her doctor prescribes Tamiflu or Relenza, she should start treatment as soon as she can.

"The safest thing she can do for herself and her pregnancy is to take the medication order to prevent an overwhelming infection," Phillippe says. "And when the swine flu vaccine comes out, pregnant women need to be among the high-risk groups that receive the vaccine first."

Especially outside the U.S., media have trumpeted advice that pregnant women should avoid crowds and basically hide out during the flu pandemic. That is NOT what women should do.

"Pregnant women should continue their everyday activities and not take any extra precautions other than avoiding people who have the infection and washing their hands frequently," Jamieson says. "But if they suspect they may have the flu, they do need to promptly call their health care provider."